Low and medium temperature source fluids, hereinafter termed source fluids of the type described, are those fluids with a temperature less than about 177° C. (350° F.), such as geothermal fluids obtained from many production wells, and industrial liquids produced by various industrial processes. The East Mesa Development Project located in the Imperial Valley of Southern California near Holtville has been producing about 4 million pounds per hour of geothermal fluid at about 162° C. (324° F.). Such geothermal fluid is an example of source fluid of the type described.
Electricity is generally produced from source fluids of the type described using a closed Rankine cycle heat engine whose operating fluid is an organic fluid (e.g., Freon), such system being termed a power plant of the type described. A source fluid of the type described is applied to a vaporizer of a power plant of the type described containing liquid organic fluid whereby the latter is converted into a vapor. The vapor is expanded in a turbogenerator that converts some of the heat in the vapor to work and produces heat depleted organic vapor that is condensed in a condenser. The condensed organic fluid is returned to the vaporizer, and the cycle is repeated.
The condenser rejects the remaining heat in the heat depleted vapor into ambient air, if an air-cooled condenser is involved, or into cooling water, if a water-cooled condenser is used. Typically, the vaporizer is operated at a pressure that produces saturated or only slightly superheated vapor because the pressures involved are relatively low and the design of the heat exchanger that constitutes the vaporizer, the piping for conveying the vapor, and the turbine, are simplified. In order to maximize power output of a power plant of the type described, the temperature drop of the source fluid across the entire heat exchanger system of the power plant, and the evaporization temperature in the vaporizer must be optimized.
Prior art cascaded power plants utilizes a plurality of closed Rankine cycle power plant modules each having an associated heat exchanger, the source fluid being serially applied to the heat exchangers of each module. Whatever system is used, maximizing the net power produced by the system is of paramount importance. One technique for increasing the power is to extract more heat from the source fluid by increasing its temperature drop. With either a single stage or cascaded system, however, increasing the amount of heat extracted from the source fluid by increasing the temperature drop of the source fluid across the heat exchanger system has the effect of decreasing efficiency of the power plant because the mean temperature of the source fluid is reduced. This results in a reduction of the evaporization temperature of the operating fluid in the heat exchanger, thus reducing the Carnot efficiency of the power plant.
In another method for increasing the efficiency level of a power plant, a prior art power plant is operated by serially applying the source fluid to the vaporizers of the modules for producing heat depleted source fluid. A preheater is provided for each vaporizer, and the heat depleted source fluid is applied to all of the pre heaters in parallel.
In an effort to increase the efficiency of a power plant of the type described, and to extract more power from the source fluid, it has been proposed to operate at super critical temperatures and pressures. In such case, the temperature of the vaporized organic fluid produced by the heat exchanger system is higher than in the above-described typical Rankine cycle power plant. While this approach is effective to increase the efficiency of the power plant and to increase its work output, the gains are offset by the higher cycle pump power consumption, as well as increased cost and complexity of the power plant whose pressure vessels must be designed to operate at pressures in the range of 500-600 psia.
Consequently, the present invention provides a cascaded closed Rankine cycle power plant using low and medium temperature source fluid which advantageously can produce an increased power level relative to that produced by prior art power plants.
Other advantages of the invention will become apparent as the description proceeds.